Examples of a conventional vibration suppressing method include “a method of suppressing a torsional vibration in an electric motor speed control system” disclosed in Patent Document 1.
FIG. 4 is a mechanical diagram showing a general variable speed driving device, and a rotating torque is transmitted to a machine 5 through a driving shaft 4 having a torsional rigidity rate K (Kgm/rad) by means of an electric motor 2. A variable speed control device 1 controls the speed of the electric motor 2 by setting, as a speed feedback signal, a signal detected by a speed detector 3 attached to the electric motor 2.
FIG. 5 is a block diagram showing the structure of a speed control device which is constituted by a digital calculating unit or an analog calculating unit as an example of the conventional art. In FIG. 5, a mean value in a constant cycle ts is calculated for the signal of the speed detector 3 (see FIG. 4) attached to the driving shaft of the electric motor by means of a mean speed calculator 23. Referring to a method of calculating a mean speed every constant cycle, if the speed detector is a pulse signal generator, for example, it is possible to obtain a mean speed in the cycle ts of a pulse signal output from the speed detector 3 as the mean frequency of a pulse in ts with a division of a count value in ts by ts. In case of a speed detecting generator of an analog type, the signal of the speed detector can be read at n times in the constant cycle ts and 1/n of the total value of these signals can be set to be the mean speed in ts.
An electric motor mean speed for each constant cycle is indicated as NMAVG and the operation of a speed controller 11 will be first described.
When a speed command NREF, the electric motor mean speed NMAVG are input to the speed controller 11 by which a speed deviation signal thereof is obtained, a signal that is obtained by adding a signal, being acquired by multiplying the speed deviation signal by a proportional gain A, and a signal, being acquired by integrating the signal in a time constant τ1, is output as a torque command signal TRFA. In the case in which the speed controller 11 has only the proportional gain A, a signal obtained by multiplying the speed deviation signal by A is output as TRFA.
Next, description will be given to the operation of an electric motor acceleration torque calculator 24. When the electric motor mean speed NMAVG for each constant cycle is input to the electric motor acceleration torque calculator 24, a signal TMAFB obtained by multiplying a value acquired by differentiating NMAVG by an inertia time constant τM of an electric motor portion is output. The signal TMAFB becomes an acceleration torque signal of the electric motor.
The operation of an electric motor acceleration torque controller 25 will be described below.
The torque command signal TRFA of the speed controller 11 is set to be the acceleration torque command of the electric motor and the output signal TMAFB of the electric motor acceleration torque calculator 24 is fed back as the acceleration torque signal of the electric motor in response to the command. When a deviation between the two signals is input to the electric motor acceleration torque controller 25, the electric motor acceleration torque controller 25 outputs a signal TRFL obtained by adding a signal acquired by multiplying the signal of the deviation between the two signals by a proportional gain G1 and a signal acquired by integrating the signal in a time constant τL1 is output to carry out a control in such a manner that the electric motor acceleration torque signal TMAFB is coincident with the torque command signal TRFA of the output of the speed controller 11.
A signal obtained by adding the torque command signal TRFA of the output of the speed controller 11 and the output signal TRFL of the electric motor acceleration torque controller 25 is set to be a torque command TRFM to carry out the torque control of the electric motor. For the load torque τTL1 to be the disturbance of the acceleration torque of the electric motor, consequently, the control is carried out in a canceling direction through the torque command compensating signal TRFL of the output of the electric motor acceleration torque controller 25. As a result, the electric motor torque command signal acts in such a direction as to cancel the disturbance torque of the electric motor which is generated by the torsion of the driving shaft during an acceleration or the sudden change of a load so that a torsional vibration is suppressed.
[Patent Document 1]
U.S. Pat. No. 3,173,007 Publication (Pages 6 to 7, FIG. 1, FIG. 9)